Electric current inverting system



May 2, 1933. E. KERN ELECTRIC CURRENT INVERTING SYSTEM Filed Oct. 14, 1931 the arc on the respective anodes.

Patented May 2, 1933 UNITED STATES PATENT OFFICE ERWIN KERN, 0F ENNETBADEN, SWITZERLAND, A SSIGNOR T0 .AKTIENGESELLSCHAFT,

SWITZERLAND, A JOINT STOCK COMPANY OF BROWN BOVERI ET CIE, OF BADEN, SWITZERLAND ELECTRIC CURRENT Application filed October 14, 1931, Serial No.

This invention relates to improvements in a system for inverting direct current into alternating current and particularly to means for improving the stability of such systems when controlled electric valves are used as the current inverting means.

Electric power systems for inverting direct current into alternating current by the use of a controlled inverter of the arcing metallic vapor type require that the ignition point of the operating anodes for the inverter be displaced when'voltage variations occur during operation of the inverter for the purpose of causing quenching of the are at the anodes in the proper sequence to secure the inverting action. Such quenching requires that the anode be without current for a certain period of time and that a. potential which is negative with respect to the cathode be impressed, at the proper time, on the grids controlling the attachment of I It is also very important that the ignition point be displaced by a greater amount than the displacement of the no-load point when variations occur in the operating voltages. To secure proper operation of the system it is desirable that the ignition point for the anodes be automatically displaced dependent on the ratio of the supplied direct voltage to the produced alternating voltage.

It is, therefore, among the objects of the present invention to provide a system for inverting direct current into alternating current by the use of an inverter of the metallic vapor type in which the ignition point or the point at which the arc attaches to the anodes, is displaced automatically in dependence on the ratio of the direct current to be inverted to-the alternating current produced. I v

Another object of the invention is to provide a system for inverting direct current into alternating current by the use of an inverter of the arcing metallic vapor type in which the ignition point of the anodes is displaced by a greater amount than the displacement of the no-load point due to voltage variations.

Another object of the invention is to pro- INVERTING SYSTEM 568,706, and in Germany September 10, 1930.

vide a system for inverting direct current into alternating current by the use of an inverter of the arcing metallic vapor type with grid control of the inverter anodes in which the ratio of the direct voltage impressed upon the grid control system to the phase voltage of the grid transformer is equal to the ratio of the direct voltage supplied to the inverter to the alternating voltage produced thereby. I

Objects and advantages other than those above set forth will be apparent from the following description when read in connection with the acompanying drawing forming a portion of the present specification and in which drawing:

Figure 1 diagrammatically illustrates one embodiment of a system in which a controlled electric valve of the metallic vapor arcing type is used to invert direct current into alternating current;

Figure 2 schemmatically illustrates portions of a. modified embodiment of an inverting system similar generally to that shown in Figure 1;

Figure 3 is a diagram illustrating the operation of the system and the voltage relations secured therein and pertaining thereto; and r Figure 4 is a diagram illustrating the operation ofthe grid control portions of the system.

Referring more particularly to the drawing by characters of reference, the reference numeral 6 designates one bus bar of a direct current supply or transmission line connected with the neutral point of a star connected "winding 7 of a transformer. Each of the phases of the transformer winding 7 are connected with one of a plurality of main anodes 8 of aninverting device 9 which is similar to a metal inclosed metallic vapor electric power rectifier of the arcing type in which the vapor is produced from a vaporizable material retained therein such as a pool of mercury at 10. The vaporizable cathode 10 is connected through a resistance 13 to a bus bar 14 forming the negative side of the direct current supply line.

ICllt transformer The direct current, inverted into alternating current by the device 9, is transmitted through the transformer primary winding 7 and a secondary winding 16 thereof to a polyphase alternating current line 17 from which it is distributed to points of use. A synchronous machine 19, which is preferably an over-excited motor or generator, is connected with the line 17 and is used for the purpose of giving the alternating current flowing in the line the proper wave shape and the proper frequency. A transformer 21 connected with the line 17 supplies a pair of excitation anodes 22 and 23 through load resistances 24.- and 25. The excitation anodes serve to maintain an are within the inverter after such are has been struck by an ignition system similar to that of the usual electric power rectifier of the metallic vapor type which is, therefore, not shown. le midpoint of the secondary winding of transformer 21 is connected through a resistance 26 and an inductance 27 with the ne ative direct current bus bar 14 as is usual in current rectifying systems.

Each of the anodes 8 of the inverter 9 is provided with a grid 30 by which the moment of transfer of the arc, from one to another of the main anodes 8, may be controlled. Each grid 30 is connected through a current limiting resistance 31 with the secondary winding of a current transformer 32 which secondary winding has a resistance 33 connected in parallel therewith. The secondary windings of the current transformers 32 and the resistances 33 are connected to the negative direct current bus bar 1% beyond the resistance 13. The primary windings of the current transformers are connected, at one end, to the secondary phase windings of a potential transformer 35, which transformer is connected through a phase shifter 36 with the alternating current line 17 and the other ends of the cur primary windings are connected through electric valves 38 with adjustable voltage dividers 39 connected across the direct current line 6, let which voltage dividers therefore, cause the cathode of the valves 38 to be at a voltage which is positive with respect to the negative direct current bus bar 14. The secondary phase windings of the transformer 35 are connected in star and the neutral point of the windings is connected with the negative bus bar 14 of the directcurrent supply line. The phase shifter 36 permits any desired adjustment of the output alternating current potential of the inverter by the voluntary displacement of the ignition point or the point in the wave cycle at which the are transfers from one main anode 8 to another of such anodes. It will be understood that the numher of main anodes, grids and grid control circuits is equal to the numrooaalo ber of phases of the transformer primary winding 7 even though only a portion of the above have been shown to simplify the explanation of the structure of the system and its operation.

hen the system is in normal operation with a normal load on both the direct current and alternating current sides of the system, a flow of current takes place from the alternating current line 17 through the transformer 35, the primary windings of the current transformers 32, the valves 38 and the voltage dividers 39 back to the neutral point of the secondary winding of the potential transformer 35. The valves 38 will conduct current when the voltage impressed thereon is greatc than the cathode potential. Every time a valve 38 conducts current, a positive voltage impulse is transmitted to the "rid 30 which causes transfer of the are from one of the main anodes to another of such anodes. In the interval between two positive voltage impulses the grids 30 have a potential impressed thereon which is negative with respect to the potential of cathode 10 because the grids are connected with the cathode through resistance 13. If the voltage changes on the alternating current side of the system the moment at which the arc attaches to any one of the main anodes is thus displaced dependent on the voltage in the direct current side of the system.

In cases of overload or short circuit on the alternating current side of the system, the direct current side of the system must be momentarily cut off from the alternating current side. Reconnection of the direct current side of the system must be. delayed until the overloaded or short circuited portion of the alternating current side has been disconnected. During such periods of overload or short circuits the alternating current voltage drops to a fraction of the voltage of the direct current network and the valves 38 are automatically blocked, until the opcrating value of the alternating current voltage is again attained. If the amplitude of the alternating current voltage is less than the amplitude of the direct current voltage in the inverter. the valves 38 cannot pass any current and thus prevent the main anodes from operating imtil the short circuit on the alternating currentside of system has been cleared and the alternating current voltage has again reached its operating value and given the required wave shape by the synchronous machine 19.

The resistance 13 serves the purpose of artificially making the voltage characteristic curve of the inverter steeper than would otherwise be the case for the reason that an additional negative bias is impressed on the grids 30 which bias is proportional to the load current passed through the inverter so resistance voltage drops A condition of the that the ignition point is automatically displaced in a lagging direction with an increase in the load. A constant E. M. F. independent of the alternating current side of the system, such as the battery 41 shown in Fig. 2 may also be connected between the negative direct current bus bar 14. and the grids 30 for the purpose of maintaining a negative bias on the grids.

Figs. 3 and 4 are diagrams which illustrate the operation of the system above described. As well known in the art of direct current inversion a main anode 8 of the inverter 9 must ignite at a point in the cycle of operations at which the corresponding phase alternating current voltage E is negative and lower in magnitude than the direct voltage E which is in opposition thereto and the ignition therefore takes place at a moment corresponding to a point such as the point A at which point, as may be seen from the drawing, the alternating current voltage E is less than the operating input voltage of the inverter designated by the line RS, the line MN designating the zero line of the voltage of the inverter. Neglecting and are drops present in the system, the main anode 8 will continue to operate until a momentcorresponding to a point B such that the area ACD equals the area DEB. To obtain continuous operation of the system anode 8 must again be without current not later than at a moment in the alternating current cycle at which the alternating current voltage decreases to a value equal to the directvoltage such as represented by the intersection of the alternating current sine curve line with the point F of the direct current line RS and the grid 30 associated with the anode 8 must then have a potential impressed thereon which is negative with respect to the cathode 10. If the ignition point A of the anode is displaced to the point D the inverter will operate at noload. Advancement of the ignition point A relative to the no-load point D corresponds to the increase of the load on the inverter if the ignition point A is advanced to such extent as to result in displacement of the point B to the right of the point F due to the constant equality between the areas ACD, BDE and the quenching of the arc flowing from the an ode 8 will not be possible and a short circuit direct current circuit will result. The alternating voltage produced by the inverter and delivered by the line 17 to points of use has been indicated in Fig. 3 by a full line sine curve Gr but it will be understood that such voltage does not conform to a true sine wave but is more accurately represented by the dotted line curve H which materially departs from a true sine curve.

Should the load on the line 17 increase tained. The ratio E /E may from the no load value to substantially full load, the alternating current voltage will decrease with respect to the magnitude of 'the'voltage of line6, 14, due to the voltage creasing the area DBFED; and as pointB is always such that A01) and BBE are equal, the described displacements of D and I may cause area DBE to become equal to area DBFED and thus cause point B to coincide with'point F and thus cause short circuit of .the line 6, 1a through the inverter 9. Such condition will not, however, occur if the control of the grids 80 be such as to cause displacement of the ignition point A in the same direction and to the same'or greater 1 extent than the displacement of point D.

The voltage relations of the grid circuits controlling the operations of the main anodes 8 of the inverter are particularly shown in Fig. 4. An anode 8 may take the are from the preceding anode at the point I, at which the valve 37 anode potential curve crosses the line OP which represents the voltage of the top of the variable voltage divider 89 above the voltage of line conductor 14 represented by line MN in Fig. 4, only if the main anode potential is greater than the potential of the cathode 10 by more than thevarc drop as previously described. It the voltage drops on the alternating current side of the system, the ignition point I for the grids as well as the ignition point for the main anodes is displaced to the right and transfer of the are from one main anode to another of the main anodes is delayed. The same ignition point displacement also takes place if the :direct voltage increases and is therefore similar to the results occurring when a voltage decrease takes place .on the alternating current side. If the ratio :E35/E39 is so held as to be equalto the ratio E /E a certain displacement of the point D causes the same displacement of the point I and shows that the transformer 7 16 and the transformer 35 i have proportional voltages.

There are two methods by which a displacement, in addition to displacement of the ignition point shown above, may be obbe made larger than E /E so that point I comes closer to the peak value of E where the rise in the curve of the alternating current voltage becomes less. The displacement of the point I caused by a voltage variation then becomes greater than the displacement of the point D. Another method by which the above reresult may be obtained consists in operating the main transformer 7/16 at a degree of saturation greaterthan the degree of saturatrolling tion of the grid transformer 35. If the main transformer is more nearly saturated, than the grid transformer, the voltage curve E will become steeper as shown by dotted line H in Fig. 3, than the grid transformer curve shown in full line K in Fig. 4. A voltage variation will therefore cause a lesser displacement of the point D than of the point I if the ratios B /E and E /E are of equal value.

It will thus be seen that a control system for an inverting device for inverting direct current into alternating current of a suitable number of phases and voltage according to the present invention produces an automatic displacement of the ignition point at which the arc is transferred from one. to another of the main anodes, and that such ignition point displacement may be produced first by suitable choice of the relations of the potential in the different portions of the circuit and second that the saturation of one transformer may be made so much greater than the saturation of another transformer that the no-load point of the inverting device will be displaced a esser distance than the ignition point.

Although but a few embodiments in the present invention have been illustrated and described it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

The invent-ion claimed is:

1. In a system for inverting direct cur rent into polyphasc alternating current, a direct current supply line a multi-anode electric valve of the metallic vapor arcing type connected with said supply line, a polyphase alternating current distribution line, a transformer connecting the anodes of said valve with said distribution line, grids controlling the transfer of the are from one of the anodes to another of the anodes, and a system for controlling the operation of said grids and including voltage dividers connected across said supply line, a potential transformer connected with said distribution line, means for shifting the phase of the voltage applied t said potential transformer, and means interconnecting said voltage dividers with said potential transformer to apply a positive voltage impulse to said grids only when the accompanying anodes are to operate.

2. In a system for inverting direct current into polyphase alternating current, a direct current supply line, a multi-anode electric valve of the metallic vapor arcing type connected with said supply line, a polyphase alternating current distribution line, a transformer connecting the anodes of said valve with said distribution line, grids conthe transfer of the are from one of the anodes to another of the anodes, and a system for controlling the operation of said grids and including voltage dividers connected across said supply line, a potential transformer connected with said distribution line, means for shifting the phase of the voltage applied to said potential transformer, means interconnecting said voltage dividers with said potential transformer to apply a positive voltage impulse to said grids only when the accompanying anodes are to operate, the degree of saturation of the first said transformer being equal to the degree of saturation of the second said transformer.

3. In a system for inverting direct current into polyphase alternating current, a direct current supply line, a multi-anode electric valve of the metallic vapor arcing type connected with said supply line, a polyphase alternating current distribution line, a transformer connecting the anodes of said valve with said distribution line, grids controlling the transfer of the are from one of the anodes to another of the anodes, and a system for controlling the operation of said grids and including voltage dividers connected across said supply line, a potential transformer connected with said distribution line, means for shifting the phase of the voltage applied to said potential transformer, current transformers connected with said potential transformer, and electric valves having cathodes connected with said voltage dividers and anodes connected with said current transformers, said grids being supplied with current from the secondary windings of said current transformers.

4. In a system for inverting direct current into polyphase alternating current, a direct current supply line, a multi-anode electric valve of the metallic vapor arcing type connected with said supply line, a polyphase alternating current distribution line, a transformer connecting the anodes of said valve with said distribution line, grids controlling the transfer of the are from one of the anodes to another of the anodes of said valve and a system for controlling the operation of said grids and including voltage dividers connected across said supply line, a potential transformer connected with said distribution line, means for shifting the phase of the voltage applied to said potential transformer, current. transformers connected with said potential transformer, and electric valves having nthodes connected with said voltage dividers and anodes connected with said current transformers, said grids being supplied with current from the secondary windings of said current transformers, the voltage supplied to said grids having a component proportional to the voltage of said supply line.

5. In a system of the character described,

a direct current supply line, an alternating current distribution line, means comprising a vapor type electric valve interconnecting said lines and operable to transform direct current supplied thereto from said supply line into alternating current and the supply thereof to said distribution line, control electrodes sequentially operable to control flow of said current through said valve, means including a voltage divider connected across said supply line interconnecting said lines operable to periodically and sequentially impress potential of such sign and magnitude on said electrodes as to cause said current to flow through said valve, and means for impressing on said electrodes potential of such sign and magnitude as to prevent said current transforming operation by said valve.

6. In a system of the character described, a direct current supply line, an alternating current distribution line, a single-cathode multi-anode electric current rectifier-inverter valve of the vapor type interconnecting said lines and operable to transform direct current supplied thereto from said supply line into alternating current and the supply thereof to said distribution line, electrodes associated with said anodes operable to control the flow of said current sequentially be tween the respective said anodes and said cathode, and means including electric valves interconnecting said lines sequentially operable in dependence upon the ratio of the voltage of said supply line to the voltage of said distribution line to periodically and sequentially impress potential of such sign and magnitude on said electrodes as to cause flow of current between the associated ones of said anodes and said cathode.

7. In a system of the character described, a direct current supply line, an alternating thereof to said distribution line, control electrodes sequentially operable to control flow of said current through said valve, voltage dividers connected across said supply line, and electric valve means interconnecting said lines by way of said dividers periodically and sequentially operable responsive to and in dependence upon the ratio of the voltage of said supply line to the voltage of said distribution line to periodically and sequentially impress potential of such sign and magnitude on said electrodes as to cause said current transforming operation of the first said valve.

9. In a system of the character described, a direct current supply line, an alternating current distribution line, means comprising a vapor type electric valve interconnecting said lines and operable to transform direct current supplied thereto from said supply line into alternating current and the supply thereof to said distribution line, control electrodes sequentially operable to control flow of said current through said valve, voltage dividers connected across said supply line, electric valve means interconnecting said lines by way of said voltage dividers periodically and sequentially operable responsive to and in dependence upon theratio of the voltage of said supply line to the voltage of said distribution line to periodically and sequentially impress potential of such sign and magnitude on said electrodes as to cause said current transforming operation by the first said valve, and means for impressing on said electrodes potential of such sign and magnitude as to prevent said current transforming operation by the first'said valve.

In testimony whereof I have hereunto subscribed my name this 30th day of September A. D. 1931. I

' ERWIN KERN.

current distribution line, means comprising a vapor type electric valve interconnecting said lines operable to transform direct current supplied thereto from said supply line into alternating current and supply thereof to said distribution line, control electrodes sequentially operable to control flow of current through said valve, and means including electric valves interconnecting said lines sequentially operable responsive to and in dependence upon the ratio of the Voltage of said supply line to the voltage of said distribution line to periodically and sequentially impress potential of such sign and magnitude on said electrodes as to cause said current transforming operation of the first said valve.

8. In a system of the character described, a direct current supply line, an alternating current distribution line, means comprising a vapor type electric valve interconnecting said lines and operable to transform direct current supplied thereto from said supply line into alternating current and the supply 

